Adam Stevens, a researcher at the University of Edinburgh and the UK Centre for Astrobiology, has worked on the science associated with the ExoMars mission and in preparation for it.

He explained that although methane is present on other planets, on Earth it is a signature of life.

"What we find on Earth is that ... some 90% of Earth's methane is biological, whether that's active now or from past biology that's decomposing," he said. "That means if we look at another planet like Mars ... maybe that's a sign of biology."

Stevens stressed that methane can be produced by other means, and not just life, but told CNN that the ExoMars mission will help "rule out some possibilities ... to tease out what is going on."

For example, he said, if methane was found at the poles where it's extremely cold then the methane is not produced by something that's melting.

ESA to test lander

ESA says the probe will also try to detect deposits of water ice below the surface, and help guide the selection of a suitable landing site for future missions.

One of the aims is to test a landing craft called Schiaparelli. The lander will separate from the orbiter and then using a parachute and thruster will descend to the surface.

ESA says the impact will be cushioned by a structure similar to a crumple zone in a car.

The landing site has been chosen because it contains a layer of iron oxide that on Earth almost always forms in an environment containing liquid water, ESA says.

Schiaparelli has been designed to operate for a few days and will measure wind speed, temperature, humidity and pressure.

This mission is part of a longer-term ESA project to explore the Red Planet. The next ExoMars launch is planned for 2020 and will include a rover that can drill into the Martian surface to analyze samples.

Photos:Mars Opportunity and Spirit rovers

From its perch high on a ridge, Opportunity recorded this image of a Martian dust devil twisting through the valley below. Just as on Earth, a dust devil is created by a rising, rotating column of hot air. When the column whirls fast enough, it picks up tiny grains of dust from the ground, making the vortex visible.

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Photos:Mars Opportunity and Spirit rovers

Opportunity photographed its tracks in the soft sand between the Endurance and Victoria craters, on the Meridiani Plains.

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Photos:Mars Opportunity and Spirit rovers

While traversing on and around the ancient volcanic feature called Home Plate, Spirit took many images of finely layered and more frothy looking volcanic rocks.

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Photos:Mars Opportunity and Spirit rovers

Tiny spherules pepper a sandy surface in this 3-centimeter (1.2-inch) square view of the Martian surface. Opportunity took this image while the target was shadowed by the rover's instrument arm.

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Photos:Mars Opportunity and Spirit rovers

A handout photo from NASA shows the Opportunity rover's route since landing on Mars in 2004.

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Photos:Mars Opportunity and Spirit rovers

This map shows the southward path driven by Opportunity from late December 2014 until it passed marathon distance on March 24, 2015, during the 3,968th Martian day, or sol, of the rover's work on Mars.

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Photos:Mars Opportunity and Spirit rovers

The Mars Opportunity rover (computer visualization).

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Photos:Mars Opportunity and Spirit rovers

The Mars Spirit rover. Both rovers feature a piece of metal with the American flag on the side. They are made of aluminum recovered from the site of the World Trade Center towers in New York City.

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The Europeans will be hoping for more success than on their previous mission to the surface of Mars. A British-built probe called Beagle 2, part of ESA's Mars Express program, was lost on Christmas Day in 2003 after contact failed during the descent.

Two NASA rovers, Spirit and Opportunity, arrived shortly after Beagle 2. Spirit got stuck in the Martian soil in 2009 and its mission was ended in 2011 but Opportunity has traveled more than a marathon distance of 26 miles and is still operational.

NASA says the rover will have a nuclear power source that can last at least 10 years and will include a test for producing oxygen from the Martian atmosphere -- something that might be crucial for future human exploration.

Its cameras will be able to generate 3-D maps, while instruments will try to detect and analyze organic molecules in the rocks and dust.

The rover will also be equipped with ground-penetrating radar to look for unusual features such as ice or brine.